The present invention relates to the x-ray tube art. It finds particular application in conjunction with high power x-ray tubes for use with CT scanners and the like and will be described with particular reference thereto. It will be appreciated, however, that the invention will also have other applications.
Typically, a high power x-ray tube includes an evacuated envelope or housing which holds a cathode filament through which a heating or filament current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high potential, typically on the order of 100-200 kV, is applied between the cathode and an anode which is also located in the evacuated envelope. This potential causes a tube current of electrons to flow from the cathode to the anode through the evacuated region in the interior of the evacuated envelope. The electron beam impinges on a small area, or focal spot, of the anode with sufficient energy that x-rays are generated and extreme heat is produced as a byproduct.
In high energy x-ray tubes, the anode is rotated at a high speed such that the electron beam does not dwell on only the small spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the anode that was heated by the electron beam has substantially cooled before returning to be reheated by the electron beam. Larger diameter anodes have larger circumferences, hence provide greater thermal loading.
In conventional rotating anode x-ray tubes, the envelope and the cathode remain stationary while the anode rotates inside the envelope. Heat from the anode is dissipated by the thermal radiation through the vacuum to the exterior of the vacuum envelope. It is to be appreciated that heat transfer from the anode through the vacuum is limited.
High power x-ray tubes have been proposed in which the anode and vacuum envelope rotate, while the cathode filament inside the envelope remains stationary. This configuration permits a coolant fluid to circulate in directed contact with the anode to provide a direct thermal communication between the anode and the exterior of the envelope. See for example, U.S. Pat. Nos. 5,046,186; 4,788,705; 4,878,235; and 2,111,412.
More specifically, an outer housing is provided which has the window through which x-rays emerge. The anode and vacuum envelope are rotatably mounted within the housing and displaced a significant distance therefrom. The chamber between the housing and the vacuum envelope is filled with a coolant oil. Connections are provided on the housing for withdrawing
oil, pumping it through a radiator or other cooling system, and returning the cooled oil to the housing. When x-rays are generated at the focal point on the anode, x-rays are emitted in substantially all directions. Because the anode has a high x-ray blocking power x-rays are effectively emitted over a basically hemispherical volume defined over the focal point where the electron beam from the cathode strikes the anode surface. These high energy x-rays pass through the vacuum envelope into the coolant oil. The coolant oil is highly radiation transparent such that x-rays passes through the oil in the reservoir to the window without significant attenuation.
One of the difficulties with this configuration is focal spot motion. Focal spot motion can arise from at least two sources in this tube type. A first source is a lack of alignment between the cathode bearing structure and the target axle, which is typically aligned with the target track surface. Parallel displacement of the cathode bearing and angular shift contribute to this misalignment and cause the focal spot to wander across or deviate from the track in a one per revolution period path.
Misalignment is caused primarily by assembly tolerance stack up and stresses built up during the welding process. Practically speaking, current technology dictates that although misalignment can be managed, it cannot be eliminated.
Thus, it becomes increasingly important to control misalignment, especially where smaller focal spot sizes and thinner slice widths are desired. Specifically, focal spot motion produces a larger apparent spot size and may give rise to artifacts as the spot moves in and out of the plane.
Accordingly, although the magnitude of focal spot motion is somewhat less than simple mechanical considerations would indicate due to the effect of electron optics in the tube, a significant problem is generated with respect to image reconstruction.
A second source of undesired focal spot motion is oscillation of the focal spot due to mechanical vibration of the tube. One type of vibration is torsional about the cathode bearing axis, with the magnets providing the restoring force. The plates, tubes, and axle of the cathode assembly also vibrate. It would be advantageous to reduce the magnitude of these vibrations or at least be able to realign the assembly conveniently after the vibration to control the focal spot motion.
The present invention provides a new and improved construction which overcomes the above-referenced problems and others.